1. Technical Field
The present invention relates to a method for controlling a direct call between mobile subscriber terminals in a mobile communication system, and more particularly, to a control method which can prevent an out-of-sync during a direct call between mobile subscriber terminals.
2. Related Art
In general, a mobile communication system includes a radio access unit (RAU) which is known as a base station, and a plurality of mobile subscriber terminals (MSTs) which are known as mobile stations. For communication between the mobile subscriber terminals (MSTs) or between the mobile subscriber terminal (MST) and a wire terminal, a communication path is formed between the MSTs or between the MST and the wire terminal through the RAU using a frequency hopping technique. Exemplars of mobile communication systems are disclosed in U.S. Pat. No. 4,998,290 for Frequency-Hopping Radio Communication Network issued to Olenick et al., and U.S. Pat. No. 5,361,401 for Channel Hopping Radio Communication System And Method issued to Pirillo.
In such a frequency hopping scheme, all communication units must have the same timing, and each communication unit must be synchronized with the rapid frequency hopping code of the system in order to communication with the system. Conventional synchronization techniques for a mobile communication system are disclosed, for example, in U.S. Pat. No. 4,442,527 for Synchronization Systems issued to Munday, U.S. Pat. No. 4,558,453 for Synchronization Method And Frequency Hopping Communication System issued to Mimken, U.S. Pat. No. 5,293,423 for Synchronization Method In A Mobile Radio System issued to Dahlin et al., U.S. Pat. No. 5,343,497 for Method And Device For The Synchronization Between A Base Radio Station And A Mobile Radio Station In A Digital Radiomobile System issued to Canosi et al., U.S. Pat. No. 5,568,510 for Apparatus And Method For Obtaining Synchronism Between A Base Station And A Portable Unit Arranged For Operation In A Frequency Hopping System issued to Tam, U.S. Pat. No. 5,390,216 for Synchronization Method For A Mobile Radiotelephone issued to Bilitza et al., and U.S. Pat. No. 5,428,637 for Method For Reducing Synchronizing Overhead Of Frequency Hopping Communication Systems to Oliva, Jr. et al.
During communications between the mobile subscriber terminals (MSTs), if the MST deviates from the call service area of the radio access unit (RAU) or the call channels of the RAU are all in busy states, a call between adjacent mobile subscriber terminals (MSTs) may be impossible. In order to overcome this problem, a direct call has been implemented for communication between the mobile subscriber terminals (MSTs) without passing through the RAU for all areas irrespective of the call service area of the RAU. However, accurate synchronization between the originating MST and the terminating MST has been a problem. As a result, the implementation of direct call between adjacent mobile subscriber terminals (MSTs) has been difficult.